1. Field of the Invention
The present invention relates to a manufacturing method of a superconducting wire.
2. Description of the Background Art
When a multi-core superconducting wire is manufactured by coating a superconducting wire of a copper based oxide with a metal, oxide powder is first filled in a metallic pipe made, for example, of silver to form a single-core wire, a plurality of such single-core wires are then combined and inserted into another metallic pipe made, for example, of silver to obtain a multi-core structure, the multi-core structure as an original wire undergoes processes such as wire drawing, rolling and the like to be formed into a wire shape, and thereafter the wire is sintered to attain a wire having superconductivity.
In such a manufacturing method, a gaseous component naturally exists inside the metallic pipe. Especially in a scheme in which a plurality of wires are fitted into a metallic pipe, a large number of components are included, which leads to the existence of various sources of gasses described below. Since the gaseous components expand inside the wires with increase in temperature, the wires are swollen. If the swelling occurs locally, the performance (critical current) of the affected portions is substantially degraded. If it is caused not locally but extensively and superficially, a superconducting ceramic portion inside comes to have a gap, the flow of current becomes poor, and the overall performance is lowered.
Furthermore, if a gaseous component remains between filaments in a multi-core structured wire, adhesion (electrical contact) between the filaments becomes insufficient, current is not supplied uniformly to each filament, and therefore the performance is not uniform.
For example, the sources of gaseous components which bring about the swelling phenomenon are as follows.
(1) carbon, oxygen, nitrogen and a hydroxyl group (xe2x80x94OH) chemically bonded to an oxide in oxide powder.
(2) carbonic acid gas, oxygen, nitrogen and water adsorbed to the surface of a powder particle.
(3) various gasses (such as air) existing in a space or gap (between powder particles, between inserted metallic pipes).
(4) vaporization of an oil or extraneous object attached to the inner and outer surfaces of an inserted metallic pipe and the inner surface of an outer metallic pipe into which a plurality of wires are inserted.
(5) a gas dissolved at a metallic pipe (silver particularly easily dissolves).
In order to cope with the swelling phenomenon, Japanese Patent Laying-Open No. 6-176635 fills powder in a metallic pipe in vacuum or in low humidity and puts a lid on it. Furthermore, Japanese Patent Laying-Open No. 8-50827 fills power into a silver bolt member having a hole formed therein (in the form of a lotus root) and puts a lid on it at a temperature not exceeding the temperature (130xc2x0 C.) which causes silver to recrystalize, and Japanese Patent Laying-Open No. 4-292811 fills degassed powder in a metallic pipe and draws a vacuum at room temperature before a lid is placed thereon.
Although the techniques disclosed in the above described publications are effective for preventing the swelling problem, it is difficult to perfectly suppress the swelling for the reasons described below. In the technique of Japanese Patent Laying-Open No. 6-176635, only the source (3) of swelling is removed. In the technique of Japanese Patent Laying-Open No. 8-50827 as well, only the source (3) of swelling is removed. Furthermore, there is not any concept of the surface of a silver pipe because a hole is formed in a block of silver in the publication. In the technique of Japanese Patent Laying-Open No. 4-292811 as well, the source (3) of swelling is removed and the sources (1) and (2) of swelling are partly removed by a process of degassing the powder. However, it cannot prevent a gaseous component from adsorbing again when filling powder in the pipe.
There may be a method of removing the sources (1), (2) and (3) of swelling by combining the techniques of Japanese Patent Laying-Open Nos. 6-176635 and 4-292811. However, the conventional art cannot eliminate the sources (4) and (5) of swelling. In any case, the techniques are intended for a single-core structure or a single lump of silver, and attention is paid only to a gas in powder.
In any case, a gas that vaporizes at a high temperature (part of (1) and (2) as well as (4) and (5)) for which a sufficient measure is not taken is confined in a sealed space. Therefore, it becomes more difficult for a gaseous component which may be released by chance during formation of a wire to be released, thus conversely causing many cases of the swelling phenomenon.
An object of the present invention is to provide a manufacturing method of a superconducting wire capable of preventing swelling of the wire caused by a residual gaseous component even when a multi-core structured superconducting wire is manufactured.
The manufacturing method of a superconducting wire according to the present invention includes the steps described below.
First, a plurality of first metallic pipes in which powder including at least a superconducting phase is filled and degassed are inserted into a second metallic pipe. The second metallic pipe in which the first metallic pipes are inserted is degassed at a high temperature. Then, the degassed second metallic pipe is sealed in a state depressurized lower than the atmospheric.
In the manufacturing method of a superconducting wire according to the present invention, the powder is degassed and therefore the sources (1) and (2) of a gaseous component can be removed. Since the first metallic pipes are degassed after the powder is filled, the source (3) of a gaseous component can be removed. Since the second metallic pipe is degassed as the first metallic pipes are inserted therein, the sources (4) and (5) of a gaseous component can be removed. Furthermore, since the second metallic pipe is sealed under a reduced pressure, the source (3) can be confined and the effect of degassing a constituent element can be maintained.
In addition, the degassing processing at a high temperature is effective for cleaning the surfaces of the inserted first metallic pipes and the inner surface of the second metallic pipe located outside, and for improving adhesion between the metallic pipes during wire formation and reducing gaps between the metallic pipes. Thus, superior electrical contact is attained and the gap as a cause of gas accumulation is hardly caused inside the wire.
As described above, all the causes for swelling the wire can be removed, and therefore such a superconducting wire can be obtained that can prevent degradation of critical current and does not worsen the overall performance.
In the manufacturing method of a superconducting wire, a step of preparing the first metallic pipe in which powder is filled and degassed preferably includes the steps of degassing powder including at least a superconducting phase at a high temperature, filling the degassed powder in the first metallic pipe, and degassing the filled first metallic pipe at a high temperature.
Thus, the powder and the first metallic pipes can be degassed.
In the manufacturing method of a superconducting wire, degassing of the powder, degassing of the first metallic pipes and degassing of the second metallic pipe are performed under a temperature condition of at least 400xc2x0 C. and at most 800xc2x0 C. under a pressure of at least 10 Pa and at most 105 Pa.
Thus, the degassing processing can be performed without changing the phase of the powder composition and therefore a high critical current value can be obtained.
In the manufacturing method of a superconducting wire, degassing of the powder, degassing of the first metallic pipes and degassing of the second metallic pipe are preferably performed under a temperature condition of at least 400xc2x0 C. and at most 750xc2x0 C. under a pressure of at least 102 Pa and at most 103 Pa or under a temperature condition of 400xc2x0 C. and at most 800xc2x0 C. under the atmospheric pressure.
As the temperature during degassing is higher, the effect of degassing increases. However, the temperature during degassing is limited by the powder portion. Although a gaseous component in the powder depends on the vacuum level (atmosphere), the reduced pressure atmosphere of at least 102 Pa and at most 103 Pa allows degassing to be performed without changing the phase of the powder composition at a temperature of at least 400xc2x0 C. and at most 750xc2x0 C. In the atmosphere and under a pressure of 105 Pa (approximately one atmosphere), the temperature of at least 400xc2x0 C. and at most 800xc2x0 C. is a temperature which does not change the wire performance. Furthermore, a lubricating oil commonly used for processing a silver pipe starts to vaporize at approximately 200xc2x0 C. The mobility of oxygen in silver becomes greater with increase in temperature. As the mobility becomes higher, oxygen is released from silver more easily. In this case as well, the temperature is desirably as high as possible. Therefore, with the above described pressure and temperature conditions, a gas can be released without changing the phase of the powder composition or without changing the wire performance.
In the manufacturing method of a superconducting wire, the superconducting phase included in the powder is preferably a bismuth based oxide superconducting phase.
For such a manufacturing method of a superconducting wire, the bismuth based oxide superconducting phase is particularly suitable.
In the manufacturing method of a superconducting wire, the powder before being filled in the first metallic pipes preferably includes a large amount of a 2223 phase and a small amount of a 2212 phase which are at least the superconducting phase.
The manufacturing method of a superconducting wire preferably further includes the step of providing a heat treatment to convert substantially all the superconducting phase of the powder to the 2223 phase after sealing the second metallic pipe.
By thus controlling the superconducting phase in the manufacturing process, a superconducting wire ultimately having a superior superconducting characteristic can be obtained.
In the manufacturing method of a superconducting wire, the step of sealing the second metallic pipe is preferably performed under a pressure of at most 0.3xc3x97105 Pa.
Although a higher vacuum level (low pressure) is desirable as the vacuum level for confinement, it may be at most 0.3xc3x97105 Pa (approximately ⅓ atmospheric pressure) considering volume expansion of a gas at a heat treatment temperature (850xc2x0 C.) for attaining superconductivity. In short, a gas expands three times at the heat treatment temperature (850xc2x0 C.) for attaining superconductivity, and therefore if the initial gas amount is ⅓ or less, the gas is housed in its original space with respect to its volume even if it expands in volume during the heat treatment for attaining superconductivity.
In the manufacturing method of a superconducting wire, the temperature at which the first metallic pipes are released to the atmospheric pressure after being degassed is preferably at most 200xc2x0 C.
Thus, swelling of the wire can be prevented.
In the manufacturing method of a superconducting wire, the second metallic pipe is preferably sealed at a temperature of at least 600xc2x0 C. and at most 750xc2x0 C.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.